Reduced external calcium or sodium stimulates calcium influx in Pelvetia eggs

The effect of external calcium and sodium ion concentrations on the calcium fluxes on the Pelvetia fastigiata De Toni egg was measured. Decreasing external [Ca(2+)] greatly increased the permeability of the eggs to Ca(2+); at 1 mM external Ca(2+) this permeability was 60 times as great as it was at the normal [Ca(2+)] of 10 mM. Lowering the external [Na(+)] also increased Ca(2+) influx; at 2 mM Na(+), the Ca(2+) influx was 2-3 times as great as it was at the normal [Na(+)] if choline was used as a Na(+) substitute. Lithium was less effective as a Na(+) substitute in increasing Ca(2+) influx. The extra Ca(2+) influx in low [Na(+)] seemed to be dependent on internal [Na(+)]. The Ca(2+) efflux increased transiently and then declined in low Na(+) media.